1. Field of the Invention
This invention relates to a composition including a novel catalytic metal-polymer complex which is useful in a method of manufacturing a laminate preform or a laminate. The laminate is catalytically effective for subsequent electroless metallization thereof and is useful, for example, as a substrate for printed circuit boards. In particular, the invention relates to a method in which reinforcing material is impregnated with a composition including a crosslinkable synthetic polymer complexed with a salt of a Group IB or Group VIII metal, and the complex is destroyed and the metal reduced to finely divided elemental metal during or after crosslinking of the polymer.
2. Background of the Art
Electrically insulating substrates may be made surface active to chemical metallization, that is, electroless deposition thereon of metal such as copper by two basic techniques. Electroless deposition of metals is well known in the art, see, for example, U.S. Pat. No. 3,560,257 to F. W. Schneble, Jr., et al., the disclosure of which is incorporated herein by reference. The substrate may be filled, i.e., fabricated to contain, a dispersed filler comprising a metal catalytic to the deposition of the electroless metal as is known from, for example, U.S. Pat. No. 3,549,009 to F. W. Schneble, Jr., et al. Alternatively, a substrate free of metal catalyst may have its surfaces activated by immersion of the substrate in a solution containing a salt of a Group IB or Group VIII metal as is known from, for example, U.S. Pat. No. 2,702,253 to Bergstrom or U.S. Pat. No. 3,011,920 to Shipley, or in a solvent-metal complex as described, for example, in U.S. Pat. No. 4,575,467 to Sirinyan.
Substrates fabricated to contain a small percentage catalytic metal according to the prior art, however, exhibit certain deficiencies. Catalytic metal coated fillers according to U.S. Pat. No. 3,546,009, for example, are known to yield substrates having an uneven distribution of the catalytic metal coated filler therein resulting from the relatively high specific gravity and tendency to settle of such fillers. Uneven filler distribution is known to cause bowing and twisting problems within the resulting substrate. Moreover, uneven metal distribution is known to cause varying rates of electroless metal deposition, for example, onto sidewall surfaces of throughbores in the substrate, during electroless metallization. Employing catalytic metal coated fillers additionally tends to produce a substrate surface that is polymer-rich and catalytic metal-deficient. Consequently, it is common to coat such substrates with a catalyst-containing adhesive coating. The coating step is tedious, however, and a common source of defects.
Substrates-surface activated according to the prior art by immersion in a colloidal suspension or solution containing a catalytic metal salt also exhibit certain deficiencies. Surface activation often leads to poor adhesion of the metal subsequently deposited by electroless metallization onto the surface of the substrate due to the weak adhesion of the interboundary layer of activator (e.g., palladium or nickel) to the substrate. Thus, the surface activation technique frequently includes the steps of mechanical roughening or chemical etching of the surface prior to surface activation. Such roughening is known to degrade the surface due to the undesirable formation of cavities, the presence of which tends to occlude some etchant, for example, an etchant including chromic and sulfuric acids, which then passivates that particular site and inhibits or prevents metallization. Moreover, substrates frequently have numerous throughbores of varying size provided therein. These throughbores are drilled to form through-hole circuitry upon metallization but, having high aspect ratios and/or being made of highly hydrophobic materials, the sidewall surfaces of such holes are difficult to wet during immersion and, consequently, difficult to activate uniformly.
Thus, for example, U.S. Pat. No. 4,493,861 to K. Sirinyan et al., teaches surface activation of substrates for currentless metallization by wetting the surface to be metallized with a palladium-II organic compound dissolved in a solvent, removing the solvent, and reducing the palladium-II organic compound adhering to the surface to be metallized. The organic moiety of the palladium-II organic compounds is an oligomeric, prepolymeric or polymeric compound of conjugated dienes or an unsaturated polyester. The compound is a complex formed by reaction of organic metallic compounds with the oligomeric, prepolymeric or polymeric compound, the reaction being accompanied by ligand exchange.
In U.S. Pat. No. 4,317,856 to D. Huthwelker et al., an aqueous solution of a compound of a metal of Group VIII or Group IB is added to a resin solution for impregnating an insulating material or to the adhesive of one or more adhesive layers applied to the insulating material. The aqueous solution may be, for example, a solution of palladium chloride (PdCl.sub.2) in water, which preferably contains a small amount of ammonium chloride. The aqueous solution may be added to a resin impregnating solution including a polyester resin, an epoxy resin, a polyepoxy resin, a phenol-resol resin, or a phenol-formaldehyde resin. The solution may be used to impregnate a paper web or glass-fibers to provide a prepreg which may be processed on a hot press into laminates useful as printed circuit boards.
Aqueous solutions of catalytic metal compounds are not compatible or have only a limited compatibility with synthetic polymers, however. Solvent systems have uniformity problems, particularly for thick layers where the solvent is driven off non-uniformly.
It is therefore an object of the present invention to provide an improved method of manufacturing a laminate which is catalytically effective for subsequent electroless metallization thereof and which has finely divided particles of catalytic metal, Me.sub.(0), uniformly distributed therein without any substantial settling, a concentrate and composition useful in the method, and a laminate or laminate preform manufactured by the method.